STDEV and STDDEVP are available in SQL Server. STDDEV is for sample population (N-1 in the denominator) and STDDEVP is for entire population (or if you have a large enough sample and want to consider that as the universe (N in the denominator)).

I didn't quite get the structure of your data - so not sure what std dev you are trying to calculate - whether it is the emailN or whether it is months (aggregated) or something else.